Currently, there are various types of ophthalmological apparatuses using an optical apparatus.
For instance, as an optical apparatus for observing an eye, there are used various apparatuses such as an anterior eye part imaging apparatus, a fundus camera, a scanning laser ophthalmoscope (SLO), and the like.
In particular, an optical tomographic imaging apparatus that performs an optical coherence tomography (OCT) utilizing interference phenomenon of multi-wavelength light is an apparatus capable of obtaining a tomographic image of a sample with high resolution.
This apparatus is becoming an indispensable apparatus as an ophthalmological apparatus for a specialist of retina in an outpatient field. Hereinafter, this apparatus is referred to as an OCT apparatus.
In the above-mentioned OCT apparatus, a measuring beam that is low coherence light is projected to a sample, and backscattered light from the sample may be measured with high sensitivity by using an interferometer.
In addition, the OCT apparatus may obtain a tomographic image with high resolution by scanning the sample with the measuring beam.
Therefore, the OCT apparatus may also take a tomographic image of a retina in a fundus of an eye to be inspected with high resolution, and thus is used widely for ophthalmological diagnosis of retina or the like.
In recent years, the OCT apparatus for the ophthalmological use is changing from a conventional time-domain method to a Fourier-domain method that may enable faster imaging.
In the case of high speed imaging, it is possible to prevent blurring or missing of an image due to an ocular movement such as small involuntary eye movements.
Compared with the time-domain method that obtains information for each specific depth in the eye to be inspected, the Fourier-domain method obtains information in the depth direction collectively, and hence high speed imaging may be performed.
However, in the case of the Fourier-domain method, it is known that the collective obtaining of information in the depth direction places some limitations on imaging.
In this situation, U.S. Patent Application Publication No. 2007/0086011 proposes a method of obtaining an optical tomographic image using a Fourier-domain OCT apparatus, which deals with the problem that lateral resolution of a tomographic image is limited by a depth of an object.
U.S. Patent Application Publication No. 2007/0086011 discloses an apparatus that is configured to extract information within an in-focus range of the measuring beam of the OCT apparatus so as to prevent deterioration in the lateral resolution.
On the other hand, “Improved spectral optical coherence tomography using optical frequency comb,” Opt. Express 16, 4163-4176 (2008) describes that measurement sensitivity in imaging a tomographic image depends on a position of a reference mirror in a Fourier-domain OCT apparatus. In other words, it is described that the measurement sensitivity is limited by a depth of an object.
As described above, the Fourier-domain OCT apparatus has a problem that the lateral resolution and the measurement sensitivity of the tomographic image are limited by the depth of the object.
U.S. Patent Application Publication No. 2007/0086011, as described above, discloses extraction of a high lateral resolution part from the tomographic image in obtaining a high lateral resolution tomographic image by using the Fourier-domain OCT apparatus.
However, U.S. Patent Application Publication No. 2007/0086011 does not mention the case where the object such as a fundus moves, or does not pay attention to a specific layer.
In addition, “Improved spectral optical coherence tomography using optical frequency comb,” Opt. Express 16, 4163-4176 (2008) discloses a structure of the Fourier-domain OCT apparatus in which the measurement sensitivity is hardly limited by a depth of an object, but it is complicated in its structure because a special light source called an optical frequency comb is used therein.